Make this Simple Refrigerator Door Open Alarm Circuit

This presented refrigerator door open alarm circuit which alerts you after a period of time whenever your refrigerator door is left open.

This circuit becomes very handy, because in case the door is left due to carelessness may cause significant increase in the consumption and affect the life of the fridge.

Circuit Operation

This circuit uses a photosensor LDR for detecting whether the door is open, or not. Whenever the sensor is illuminated by the light coming out from inside of the refrigerator, the circuit begins emitting an intermittent sound to alert you and bring the situation to your attention.

And as soon as the door is closed, and the fridge light goes off, the circuit and the alarm shuts off and stops emitting the sound.

For executing the entire operation a couple of timers ICs 555 are connected as shown in Figure.

When the LDR is not introduced to light the voltage on pin 2 (trigger) of the first IC 555 stays higher and its output (pin 3) is rendered low. Due to this the second IC 555 is rendered inhibited (low voltage level on pin 4) and the alarm is not allowed to activate.

When the LDR experiences an illumination (door is opened), the voltage level on pin 2 of the first 555 gets low causing the output (pin 3) to oscillate (square wave).

During the oscillation when output of the first 555 is at high level enables the second 555 to get triggered which also begins oscillating in tune with the first but at a much higher frequency.

A buzzer which may be seen connected with the output of IC2 now begins buzzing and alarming.

The circuit makes use of a PP3 9 volt battery, and should be placed as close as possible to the inner light of the refrigerator.

The circuit should be housed inside a box that may be waterproof and sealed to prevent moisture from affecting its operation.

Circuit Diagram

Parts list of the refrigerator door open alarm circuit

• IC1 - IC2: 2 Timer 555
• C1: 1uf 25V
• C2: 100nF
• R1: 10K 1 / 4W
• R2: LDR (photoresistor)
• R3: 2.2M 1 / 4W
• R4: 1M 1 / 4W
• D1:1N4148
• Buzzer: Piezo type DC

Another Simpler Design

The above fridge door open warning circuit can be much simplified as shown in the following diagram:

Here the IC 555 is configured as an astable frequency generator which creates an oscillating frequency on the connected speaker so that the speaker can emit a warning tone.

For the 555 astable to emit the sound it is necessary that pin#4 is connected to the positive line and is not grounded by the right side BC547 transistor.

However, when the fridge door is open, the internal lamp illuminates and the light falls on the LDR. The LDR resistance drops causing a voltage supply at the base of the left side BC547. This BC547 now conducts so that the base of the right side BC547 is grounded. Due to this the right side BC547 shut off.

This in turns allows the pin#4 of IC 555 to get a positive potential via R3 1K resistor.

IC 555 astable now becomes active and starts generating a warning tone on the speaker.

As soon as the fridge door is closed, the internal lamp shuts off causing a total darkness on the LDR. The LDR resistance increases drastically causing the left side BC547 to shut off.

Due to this right side BC547 switches ON and its collector grounds the pin#4 of the IC 555.

IC 555 now shuts down switching OFF the speaker warning tone.

The above concept can be further simplified as given in the following diagram:

If the circuit does not work correctly try reducing the R3 value to 10K.